Re: [PATCH][AArch64] PR79262: Adjust vector cost

[Richard Biener <richard dot guenther at gmail dot com>]

On Mon, Jan 22, 2018 at 4:01 PM, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
> PR79262 has been fixed for almost all AArch64 cpus, however the example is still
> vectorized in a few cases, resulting in lower performance.  Increase the cost of
> vector-to-scalar moves so it is more similar to the other vector costs. As a result
> -mcpu=cortex-a53 no longer vectorizes the testcase - libquantum and SPECv6
> performance improves.
>
> OK for commit?

It would be better to dissect this cost into vec_to_scalar and vec_extract where
vec_to_scalar really means getting at the scalar value of a vector of
uniform values
which most targets can do without any instruction (just use a subreg).

I suppose we could also make vec_to_scalar equal to vector extraction and remove
the uses for the other case (reduction vector result to scalar reg).

Richard.

> ChangeLog:
> 2018-01-22  Wilco Dijkstra  <wdijkstr@arm.com>
>
>         PR target/79262
>         * config/aarch64/aarch64.c (generic_vector_cost): Adjust vec_to_scalar_cost.
> --
>
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index c6a83c881038873d8b68e36f906783be63ddde56..43f5b7162152ca92a916f4febee01f624c375202 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -403,7 +403,7 @@ static const struct cpu_vector_cost generic_vector_cost =
>    1, /* vec_int_stmt_cost  */
>    1, /* vec_fp_stmt_cost  */
>    2, /* vec_permute_cost  */
> -  1, /* vec_to_scalar_cost  */
> +  2, /* vec_to_scalar_cost  */
>    1, /* scalar_to_vec_cost  */
>    1, /* vec_align_load_cost  */
>    1, /* vec_unalign_load_cost  */